Differential detector circuits



April-27, 1948. M. G. CROSBY DIFFERENTIAL DETECTOR CIRCUITS OiiginalFiled July 17, 1941 2 Sheets-Sheet 1 INVENT R flurxaya wii BY KMATTORNEY April 27, 1948. M. s. CROSBY DIFFERENTIAL DETECTOR CIRCUITSOriginal Filed July 17, 1941 2 Sheets-Sheet 2 INVENTOR MlfmgG: rod 6yATTORNEY Reiuued Apr. 27, 1948 DIFFERENTIAL DETECTOR CIRCUITS Murray G.Crosby, Itiverhead, N. Y, alsignor to Radio Corporation of America, acorporation of Delaware Original No. 2,296,092, dated September 15,1942, Serial No. 402,184, July 1'8, 1941. Application for reissueDecember 28, 1943, Serial No. 515,963

13 Claims.

My present invention relates to differential diode detector circuitsadapted for reception of frequency, or phase, modulated carrier waves oramplitude modulated carrier waves, and more particularly to suchdetector circuits which include means for readily switching from onetype of reception to the other type.

In the past the differential detector circuit has been constructed tofavor frequency modulated carrier wave (FM) reception, and for amplitudemodulated carrier wave reception a separate detector was utilized, or aspecific type of combining circuit was employed. Several methods havebeen employed to provide a detection circuit capable of utilization witheither of the aforesaid types of reception. Some of these methodsrequired the use of one or more coupling tubes, and usually had thedisadvantage that the automatic frequency control (AFC) voltage appearedas a balanced-to-ground voltage instead of the more desirable potentialagainst ground.

Accordin ly. it may be stated that it is one of the main objects of thisinvention to provide a simple type of diiferential diode detectorcircuit which eliminates the necessity of using extra tubes and producesthe desired type of AFC potential; and which can be utilized for eitherof FM or AM (amplitude modulated carrier wave) reception by theexpedient of adjusting a simple switch mechanism.

Another important object of this invention is to provide a differentialdiode detector circuit which is so arranged that, when fed from afrequency discriminator network, terminals are available for detected AMvoltage, FM voltage, automatic-volume control (AVC) and AFC voltages,and these terminals being independent of each other so that amplitudeand frequency modulated carrier waves may be detected simultaneously andAFC and AVC potentials be continuously available.

Still other objects of my invention are to improve generally thesimplicity and efficiency of differential diode detector circuits, andmore especially to provide differential detector circuits which arereadily and economically assembled in radio receivers. 1

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and 'method of operationwill best be understood by reference to the following description takeninganizations whereby my invention may be carried into effect.

In the drawings: I

Fig. 1 shows one arrangement embodying the invention,

Fig. 2 shows a modification of the embodiment of Fig. 1, a differenttype of discriminator network being employed,

Fig. 3 shows a further modification wherein still another form ofdiscriminator network is employed,

Fig. 4 illustrates a further modification wherein the diiferentialdetector is used to drive a pushpuli amplifier,

Fig. 4a illustrates a modification of a portion of the differentialdetector circuit of Fig. 4.

Referring now to the accompanying drawings, wherein like referencecharacters in the diiferent figures designate similar circuit elements,the circuit arrangment of Fig. 1 illustrates a differential diodedetector circuit which may be utilized in a superheterodyne receiver ofthe type capable of receiving FM or AM waves. It is not believednecessary to describe the networks prior to the input transformer of thedetector circuit, because those skilled in the art are fully acquaintedwith such receiver networks. It is believed sufficient to point out thatin the case of FM reception, and assuming the FM reception to cover42-50 megacycles (me). the receiver would utilize a converter network toreduce the selected FM waves to a lower frequency. The operatingintermediate frequency (I. F.) may be of the order of 4.3 mc., and theI. F. waves would be amplifled in several stages of I. F. amplificationfollowed by any well known form of amplitude modulation limiter device.The pass band of the coupling networks would depend upon the centerfrequency deviation range, and, of course, the pass band of the variouscoupling networks would be suillciently wide to pass the entire centerfrequency deviation range. Assuming that wide band FM waves areemployed, it will be understood that a pass band of substantially to 200kilocycles (kc.) will be utilized in each of the coupling transformers.

Hence, the I. F. input transformer l is shown as comprising a primaryresonant circuit I which is tuned to the center frequency of the appliedFM waves, and this center frequency is, of course, the operating I. F.value. A damping resistor is shown shunted across the circuit to providea suiliciently wide pass 'band for the FM waves. The transformer i isprovided with a pair of secondary resonant circuits II and ii. Each ofthem is provided with a damping resistor to provide a sumciently widepass band. These secondary circuits l and II are mistuned with respectto the center frequency of the applied FM waves. The mlstuning is inopposite senses, but in equal frequency amounts. Thus, tuned circuitcould be tunedto a certain number of kilocycles above the centerfrequency, while circuit ll would be tuned the same number of kilocyclesbelow the center frequency. This type of discriminator network is wellknown in the art. and it is not believed necessary to describe it in anyfurther detail.

This type of discriminator converts applied FM waves to AM waves withmodulation envelopes 180 degrees out of phase. These modulationenvelopes are detected by diodes 2 and 3 which make the detected outputsavailable across resistors 4-5 and 8-1. Resistors 4 and i are arrangedin series between the cathode of diode 2 and the low potential side oftuned circuit Ill. the I. F. by-pass condenser 8 being connected inshunt across resistors 4-5. Simultaneously, resistors I and 8 arearranged in series relation between the cathode of diode 3 and the lowpotential side of tuned circuit II, the I. F. by-pass condenser 9shunting resistors 8-4. The lunction of resistors 4 and I and theJunction of resistors 8 and 1 are directly connected together, and thecathode end of resistor 1 is established at ground potential. The mannerin which these resistors are connected together makes the detected FMvoltage available across resistors l and I.

That is to say, the FM voltage is taken off at the cathode end ofresistor 4. Hence, the letters FM are applied to a terminal connecteddirectly to the cathode end of resistor 4. It will be noted that theoutput of the two resistors l and 1 is reversed in phase so that theproper Phase reversal is accomplished for the addition of the detectedFM voltage. The slow variations of frequency, corresponding to carrieror local oscillator frequency drift rates, also appear across theseresistors 4 and 1 which are available for AFC purposes. Therefore, theAFC connection is made to the FM voltage output lead. As is well knownto those skilled in the art, the AFC bias may be employed to control thelocal oscillator of the receiver so as to vary its frequency in a senseto insure, or maintain, the correct value of I. F. v

If the receiving system is used to receive AM waves, then it is merelynecessary to connect an output terminal, designated AM. to the anode endof resistor 5. The AM voltage output is taken from across resistors'iand I. It will be noted that these two resistors 5 and I are connectedso that they both have their ends which are towards the diode plates athigh potential, and the combination of the detectedAM voltage outputs isin phase. The normal adjustments of the resistors are to make them allequal in order to effect the balanced conditions for both AM and FMreception. It is to be noted that the by-pass condensers 8 and 9 willby-pass the since it is a summation of the voltages across resistors Iand 5. The second point from which to derive AVC voltage is designatedAVC', and this output lead is connected to the anode end of resistor 8.This output point AVC' gives a control potential corresponding to thevoltage from one detector only. since it is the sum of the voltagesacross resistors 1 and 5 which are both in the same circuit with diode3. The choice of which of these two points to use to obtain the AVGpotential depends upon the exact nature of the discriminator which feedsthe diodes. For most purposes there will be little difference betweenthe types of AVG action obtained from the two points. Those skilled inthe art are fully acquainted with the manner of using the AVG bias. Itis only necessary to point out that the AVG bias is applied to theamplifiers prior to the differential detector so as to vary the gain ofthese amplifiers in a sense to compensate for carrier fading.

An adjustable switch element l 2 is provided so as to select either FMor AM voltage depending upon the type of reception which is had. Theadjustable element is connected to the input electrode of the followingaudio frequency amplifier, and the element l2 may be adjusted to contacteither the FM or AM output terminal of the differential detector. It ispointed out that when receiving AM waves with this system the AM waveswillpass through the receiving system in the usual manner. it being onlynecessary that the carrier frequency of the AM waves be of a value equalto the tuned frequency of the Drimary resonant circuit i', or statedmore generally, the carrier frequency of the AM should be located in thepass band of the transformer I.

- Simultaneous derivation of detected FM and AM I. F. currents fed tothe discriminator, but allow the modulation frequencies to pass.

There are two points from which AVC potentials may be obtained. Thefirst point is from the same point that the AM voltage output isobtained, and, hence, the letters AVC are applied to a lead connected tothe AM voltage output lead. This point gives a potential correspondingto a summation of voltages from both detectors,

voltages could be taken of! in such cases where the carrier at thetransmitter is simultaneously frequency modulated by one type of signaland amplitude modulated by another type of signal. In such a case. ofcourse. adjustable switch element l2 would be replaced by a pair ofleads connected to the respective FM and AM output P ints.

When the common carrier is frequency and amplitude modulated a smalldegree of amplitude modulation (probably not over about 50 percent)would be used for the'AM signals, and the carrier is frequency modulatedin the normal manner. Another way of detecting this type of modulationwould be to divide the signal into two branches one of which is afrequency modulation detector and the other an amplitude detector. Withsuch separate branches, the FM detector may include a limiter to morecompletely remove the amplitude modulation, but with the system shownhere, the balance of the detectors would be depended upon to cancel theamplitude modulation from the FM output. No limiter would be used priorto the detector in Fig. l for AM or simultaneous FM and AM reception.

In Fig. 2 the diodes 2 and 3 are shown connected to a different type ofdiscriminator input circuit. The discriminator inthis case is of thetype disclosed by S. W. Seeley in U. S. Patent 2,121,103, granted June21, 1938. The primary circuit is again tuned to the center frequency ofthe applied FM waves. Instead of using a single tuned secondary circuit,two secondaries l3 and I! are employed, the condenser l5 shunting thesecondaries and tuning them to the center frequency of the appliedwaves. The high potential side of circuit I is connected by a directcurrent I are again arranged in series between the cathode oi diode 2and the low potential end of coil It, a choke coil being connected inseries with resistor I.

The cathode end of resistor l is at ground potential and a choke coil 8'is in series with resistor I. The resistors l and 8 are connected inseries between the cathode diode I and the low potential end 01' coilll. The two secondaries II and II are used in-place of the usualmid-tapped secondary winding in order to isolate the rectified currentsin the two resistors adiacent the chokes. It is not believed necessaryto describe the manner in which the discriminator circuit operates inFig. 2 to produce the detected FM voltage between the cathode end 01'resistor 4 and ground. It will be noted that the various output voltageconnections fire made to substantially the same point as in the case ofFig. 1.

In Fig. 3 there is shown an arrangement wherein the discriminator inputcircuit is of the type disclosed in my U. 8. Patents 2,085,008,2,156,374, and 2,312,079. While this detector is primarily a phasemodulation (PM) detector, it will detect I'M waves ii the degree oimodulation is low enough. In this type or circuit the transformer I hasitsprimary circuit l' and secondary circuit l f' constructed andarranged so as to have a pass band equal to the channel width of theapplied waves. Each of I and l is tuned substantially to the carrierfrequency. A piezoelectric crystal ll, tuned to the center frequency ofthe applied FM waves, is arranged between two pairs oi crystalelectrodes 2| and 22 in well known manner. The pair of crystalelectrodes II is connected to oppositeends oi the secondary circuit I",while the second pair 22 is connected to the respective anodes of diodes2 and 3. This provides a self-neutralized type 01 crystal holder inwhich the diodes are connected differentially, with one diode cathodebeing grounded.

It is not believed necessary for the purposes or this application togive a detailed description 01' the manner oi operation of th dis rinator. reference being made to my aforesaid patents and application fora detailed explanation, as well as to my paper "Communication by PhaseModulation" in Proceedings of I. R. E. i'or I 'ebruary. 1939. It issuflicient to point out that the crystal functions to rotate the phaseof the vector resultant of the modulation side bands of the applied PMwaves so that the resultant vector of the modulation side bands is inphase with the carrier. By means of this phase rotation action. the PMwaves are convertedto AM waves which are rectified by the differentiallyconnected diode detectors. Specifically, resistors I and 5 are arrangedin series with a resistor 25 between the anode and cathode of diode 2.Resistors I and 0 are arranged in series relation with resistorbetweenthe anode and cathode oi diode I. Resistors and 28 provide directcurrent circuits for the diodes, and. also, act to divide the crystaloutput into two portions. Here, again, the various output voltageconnections are seen to be taken of! at the same points as in the caseof Figs. 1 and 2.

With this circuit the simultaneous reception oi phase modulation andamplitude modulation is possible, and AFC bias is simultaneouslyproduced. The usual procedure, however, is the 9.1-"

' end or resistor l. as in the case of Figs. 1 and 2 where the FM outputterminal is made to the same point. When FM waves are received by thiscircuit of Fig. 3, the S-shaped curve 01' Fig. 9

or my I. R. E. paper is depended upon for detection.

In Fig. 4 there is shown the arrangement of Fig. 1 constructed to drivea push-pull audio amplifier. There is shown the audio amplifier tubes 30and 3| each having their signal input grids arranged for Push-pulloperation, and the plate circuits are arranged for push-pull operationas well. The signal grid 32 of tube 30 is connected through the audiocoupling condenser 33 to the adjustable element II which may be adjustedto select either of the detected PM or AM voltage output terminals. Thejunction of resistors 4 and 'l is at ground potential, while the grid 4001' tube Si is connected by the audio coupling condenser ll. to thecathode end of resistor I.

It will be noted that the usual method of reversing the phase orcombination when such a push-pull audio amplifier is used makes use oi areversin switch between one of the amplifier plates and the outputtransformer, This requires an output transformer with both platewindings separated instead of the mid-tapped type shown in Fig. 4. Theprior arrangement, also, has the disadvantage that for the, amplitudemodulation connection the tubes are no longer connected in push-pull, sothat the benefit oi the direct current flux balance is not obtained. Inthe circuit arrangement oi! Fig. 4 it is possible to use a single-pole,double-throw switch to change from amplitude modulation to frequencymodulation reception.

In the circuit of Fig. 4a, it is shown how the separate resistors 45 and1-6 of the two diodes 2 and 3 may be by-passed individually. This typeof by-passing sometimes is advantageous since all points of the;resistors are then bypassed, and the mid-tap oi the upper dioderesistors is not leit unby-passed by the presence of the lower dioderesistors between the mid-tap and ground. I

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

WhatIclaim is:

1. In combination, a first diode having a modulated carrier wave inputcircuit, a pair of resistor elements arranged in series relation be 7able center frequency or modulated carrier waves or constant carrierfrequency and variable amplitude, like resistors of said two pairs ofresistors being diflerentially connected to provide thereacrossmodulation voltage corresponding to the aforesaid center frequencyvariations, means for deriving said modulation from across saiddifferentially connected resistors, and additional means connected tothe anode end of one of said pair of resistors for deriving modulationvoltage corresponding to said modulated waves of variable carrieramplitude.

2. In combination with a first diode having a modulated carrier waveinput circuit and a pair of resistors arranged in series relationbetween the anode and cathode of the diode, a second diode having amodulated carrier wave input circuit and a second pair of resistorsarranged in series relation between the anode and cathode of the seconddiode, means applying to said input circuits modulated carrier waveswhich are alternatively frequency or amplitude modulated, meansestablishing the cathode end of one of said pair of resistors at groundpotential, means connecting in series relation between the cathodes ofsaid diodes like resistors of said two pairs of resistors, means forconnecting a detected frequency modulation voltage output terminal to 8or resistors arranged in series relation between the anode and cathodeof the diode, a second diode having a modulated carrier wave inputcircuit, and a second pair of resistors arranged in series relationbetween the anode and cathode of the second diode, means applying tosaid input circuits modulated carrier waves which are alternativelyfrequency or amplitude modulated, means connecting in series relationbetween the cathodes of said diodes like resistors of said two pairs ofresistors, means grounding the junction of said like resistors, meansfor connecting detected frequency modulation voltage output terminals tothe ungrounded cathode ends of said pair of like resistors, and aseparate detected amplitude modulation voltage terminal connected to theanode end of one of said pair of resistors.

6. In combination, a first diode having a moduiated carrier wave inputcircuit, a pair of resistor elements arranged in series relation betweenthe cathode of said diode and one side of said input circuit, a se anddiode provided with a modulated carrier wave input circuit, a secondpair of resistors arranged in series relation between the cathode of thesecond diode and one side of its input circuit, a connection between thejunctions of said first pair of rethe ungrounded cathode end of saidpair of like resistors, and a separate detected amplitudemodulationvoltage terminal connected to the anode end of one of saidpair of resistors. 3. In combination, a first diode having a frequencyand amplitude modulated carrier wave input circuit, a pair of resistorelements arranged in series relation between the cathode of said diodeand one side of said input circuit, a second diode provided with amodulated carrier wave input circuit, a second pair of resistorsarranged in series relation between the cathode of the second diode andone side of its input circuit, a connection between the junction ofsaidflrst pair of resistors and said second pair of resistors, likeresistors of said two pairs of resistors being difierentially connectedto provide thereacross modulation voltage corresponding to centerfrequency variations of the modulated carrier waves, means for derivingsaid modulation from across said differentially connected resistors, andaddi-- tional means connected to the anode end of one of said pair ofresistors for deriving modulation voltage corresponding to amplitudevariation of said modulated waves.

4. In combination with a first diode having a modulated carrier waveinput circuit and a pair of resistors arranged in series relationbetween the anode and cathode of the diode, a second diode coupled tosaid modulated carrier wave input circuit, a second pair of resistorsarranged in series relation between the anode and cathode of the seconddiode, means applying to said input circuit modulated carrier waveswhich are alternatively phase or amplitude modulated, means establishingthe cathode end of one of said pair of resistors at ground potential,means connecting in series relation between the oathodes of said diodeslike resistors of said two pair of resistors; means for connectingdetected phase modulation voltage output terminal to the ungroundedcathode end of said pair of like resistors, and a separate detectedamplitude modulation voltage terminal connected to the anode end of oneof said pair of resistors.

5. In combination with a first diode having a modulated carrier waveinput circuit and a pair sistors and said second pair of resistors,means for applying to the input circuit of each of said diodes modulatedcarrier waves, like resistors of said two pairs of resistors-beingdifferentially connected to provide thereacross a first modulationvoltage corresponding to frequency variations of the carrier waves,means for deriving said first modulation from across saiddiilferentially connected resistors, and additional means connected tothe anode end of one of said pair of resistors for derivin a secondmodulation voltage corresponding to amplitude variations of appliedcarrier waves.

7. In combination with a first diode having a modulated carrier waveinput circuit, a pair of resistors arranged in series relation betweenthe anode and cathode of the diode, a second diode utilizing saidmodulated carrier wave input circuit, a piano-electrical crystal, tunedto the frequency of the said input circuit, coupling said input circuitto said diodes, a second pair of resistors arranged in series relationbetween the anode and cathode of the second diode, means applying tosaid input circuit modulated carrier waves which are alternativelyfrequency or amplitude modulated, means establishing the cathode end ofone of said pair of resistors at ground potential, means connecting inseries relation between the cathodes of said diodes like resistors ofsaid two pair of resistors, means for connecting a detected frequencymodulation voltage output terminal to the ungrounded cathode end of saidpair of like resistors, and a separate detected amplitude modulationvoltage terminal connected to the anode end of one of said pair ofresistors.

8. In combination, a first diode having a frequency and amplitudemodulated carrier wave input circuit, a pair of resistor elementsarranged in series relation between the cathode of said diode and oneside of said input circuit, a second diode provided with a modulatedcarrier wave input circuit, a second pair of resistors arrangedierentially connected to provide thereacross mod ulation voltagecorresponding to frequency variations of the modulated carrier waves,means including a push-pull amplifier for deriving said modulation fromacross said diflerentially connected resistors, and additional means,connected to the anode end 01 one of said pair of resistors, forapplying modulation voltage corresponding to amplitude variation of saidmodulated waves to said amplifier.

9. In combination with a first diode having a modulated carrier waveinput circuit and a pair of resistors arranged in series relationbetween the anode and cathode of the diode, a second diode, meanscoupling the second diode-to said modulated carrier wave input circuit,a second pair of resistors arranged in series relation between the anodeand cathode of the second diode, means applying to said input circuitmodulated carrier waves, means establishing the cathode end or one ofsaid pair of resistors at relatively invariable potential, meansconnecting in series relation between the cathodes of said diodes likeresistors of said two pair of resistors, means for connecting an outputterminal for frequency variable modulation to the ungrounded cathode endof said pair of like resistors, and a separate amplitude modulationoutput terminal connected to the anode end oi one of said pair ofresistors.

10. In combination with a first diode having a modulated carrier waveinput circuit and a pair of resistors arranged in series relationbetween the anode and cathode of the diode, a second diode having amodulated carrier wave input circuit, and a second pair of resistorsarranged in series relation between the anode and cathode of the seconddiode, means applying to said input circuits modulated carrier waves,means connecting in series relation between the cathodes 01 said diodeslike resistors 01 said two pair 01' resistors. means grounding theJunction of said like resistors, means for connecting detected irequencymodulation voltage output terminals to the ungrounded cathode ends ofsaid pair of like resistors, separate detected amplitude modulationvoltage output terminals connected to the anode end of oneot said pairof resistors, and a push-pull amplifier having input terminals adaptedto be connected to either of said output terminals.

11. In a detector oi signal-modulated carrier energy, afrequency-responsive network comprising two circuits havingfrequency-response characteristics of the same type but sloping inopposite senses with irequency over a predetermined range of frequencydeviation of a carrier signal applied thereto, a pair of rectifiersindividually coupled to said circuits, aload impedance in circuit withone of the two circuits and one rectifier, a second separate loadimpedance in circuit with the second circuit and the second rectifier,fractional portions of said impedances being in series relation, andmeans for separately deriving from diflerent points of said loadimpedances difier- ,ence and summation components respectively ofseparately rectified outputs of said rectifiers.

12. In combination with a source of signals, a pair 01' detectors eachhaving a respective input circuit coupled to said source, a respectiveoutput resistor operatively associated with each detector, means forjoining a point on one resistor to a point on the other'resistor, saidpoints being located so as to divide each output resistor into first andsecond sections, connections for deriving from across the first sectionof one output resistor and the second section oi. the second outputresistor in series aiding relation the additive combination of thedetected voltages developed thereacross, and separate connections forderiving from across each of the second sections of the two outputresistors in series-opposing relation the differential combination ofthe detected voltages developed thereacross.

13. In a balanced frequency modulation detecting system, a pair ofdetectors each provided with its respective output resistor, meansjoining the midpoints of said resistors thereby to divide each outputresistor into respective first and second sections. connections toderive detected frequency modulation signal voltage from thedifierential of the detected voltages developed across the respectivesecond resistor sections of said output resistors in series-opposition,and connections to derive detected amplitude modulated signal voltagefrom the addition 0! the detected voltages developed across the firstsection of one output resistor and the second section of the secondoutput resistor in series-aiding relation.

MURRAY G. CROSBY.

